"Prices and availability are updated every 24 hours.
Seat availability may be extremely limited at the prices quoted, and some fares may include journeys that involve connecting flights or services operated by our airline partners."

seems like a case of sold out for today - but in theory within 24 hours they should either release another seat at this price or change the minimum fare for the month to the higher level.

I guess one seat a day at £268 each way would do it....